1. Field of the Invention
The present invention relates generally to an alkaline secondary battery including a positive electrode, a negative electrode and an alkaline electrolyte solution, and particularly to an alkaline secondary battery which employs zinc as a negative electrode material for use in the negative electrode and which provides satisfactory charge/discharge cycle performance in a case where the positive electrode is adapted to present a smaller capacity than the negative electrode at least in an initial charge/discharge period and the negative electrode contains zinc in an increased proportion for increase in the battery capacity.
2. Description of the Related Art
Heretofore, the alkaline secondary batteries have employed various materials as the negative electrode material for use in the negative electrode.
An alkaline secondary battery of high energy density is obtained by the use of zinc with a small electrochemical equivalent and electrode potential as the negative-electrode material. In this connection, various studies have long been made on the alkaline secondary batteries employing zinc as the negative-electrode material.
There has been disclosed in Japanese Examined Patent Publication No.45(1970)-3570 an alkaline secondary battery of a so-called inside-out construction which includes a cylindrical positive electrode with a central cavity, and a negative electrode comprised of a bar-like negative-electrode current collector and a zinc layer, as the negative-electrode material, formed around the current collector, the negative electrode received by the central cavity of the positive electrode (so called outside-positive electrode cell).
The alkaline secondary battery of such an inside-out construction features a high energy density by virtue of a great amount of negative-electrode and positive-electrode materials contained in the battery. Unfortunately, repeated charge/discharge cycles result in shortage of the alkaline electrolyte solution in the zinc based negative electrode. Consequently, the battery is lowered in the charge/discharge cycle performance.
As a solution to this problem, Japanese Examined Patent Publications No.45(1970)-4254 and No.45(1970)-17332 have proposed alkaline secondary batteries of inside-out construction which each include a cavity for holding the alkaline electrolyte solution or water to be fed to the negative electrode, thereby achieving improved charge/discharge cycle performance.
However, in a case where the cavity for holding the alkaline electrolyte solution or water is disposed above the negative electrode as suggested by Japanese Examined Patent Publication No.45(1970)-4254, the negative electrode is not sufficiently supplied with the alkaline electrolyte solution at a portion around the negative-electrode current collector which is spaced away from the positive electrode. Consequently, the battery fails to achieve an adequate improvement in the charge/discharge cycle performance.
It is to be noted that the alkaline secondary batteries disclosed in the above official gazettes employ manganese dioxide as the positive electrode material for use in the positive electrode.
Unfortunately, manganese dioxide has a poor reversibility of the charge/discharge reaction process. Hence, such an alkaline secondary battery normally restricts the capacity of the zinc based negative electrode within the range of a chargeable/dischargeable capacity of the manganese-dioxide based positive electrode. As a result, a satisfactory battery capacity cannot be obtained.
On this account, the current trend of the alkaline secondary battery is to use nickel hydroxide as the positive electrode material for use in the positive electrode. In a case where nickel hydroxide, having a superior charge/discharge characteristic to manganese dioxide, is used as the positive electrode material, the capacity of the positive electrode may be restricted within the range of a chargeable/dischargeable capacity of the zinc based negative electrode. Thus, the satisfactory charge/discharge capacity may be obtained by increasing the amount of zinc contained in the negative electrode.
The charge/discharge reaction process occurring in the battery having the nickel-hydroxide based positive electrode is expressed by the following chemical formula, wherein the discharge reaction process involves the loss of water in the alkaline electrolyte solution: EQU Ni(OH).sub.2 +Zn(OH).sub.2.revreaction.NiOOH+Zn+2H.sub.2 O
Accordingly, the alkaline secondary battery employing nickel hydroxide as the positive electrode material has a detrimental tendency of suffering shortage of the alkaline electrolyte solution during the discharge process. In the negative electrode, on the other hand, a zinc dissolution/deposition process takes place in conjunction with the charge/discharge reaction process. Thus, in conjunction with repeated charge/discharge processes, the repeated zinc dissolution/deposition processes cause gaps between zinc particles to become blocked and passivated. This interferes with permeation of the alkaline electrolyte solution into the negative electrode, resulting in a battery capacity decline. Particularly, in the case of the alkaline secondary battery of inside-out construction with the negative electrode containing the increased amount of zinc for achieving the sufficient charge/discharge capacity, a serious decline in the battery capacity results from the zinc passivation resulting from the charge/discharge process. As a result, the alkaline secondary battery is lowered in the charge/discharge cycle performance.